1. Field of the Invention
The present invention relates to nonaqueous electrolyte secondary batteries, and more particularly relates to a nonaqueous electrolyte secondary batteries with excellent continuous charge characteristics.
2. Description of Related Art
Portable information terminals such as mobile phones, notebook personal computers, and PDAs are becoming increasingly smaller and lighter in recent years. In line with this trend, secondary batteries used as power sources for driving them are required to have higher capacities.
The present most widely used nonaqueous electrolyte secondary batteries have a positive-electrode active material composed of a lithium-containing transition metal oxide and a negative-electrode active material composed of a graphite-based carbon material. Lithium-containing transition metal oxides are favorable to provide smaller and lighter batteries. Most of these nonaqueous electrolyte secondary batteries have a nonaqueous electrolytic solution containing a lithium salt such as LiPF6 dissolved in a solvent mixture of a cyclic carbonate ester and a chain carbonate ester. Examples of the cyclic carbonate ester include ethylene carbonate (EC), and examples of the chain carbonate ester include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).
These batteries are required to have higher charge voltages to increase their capacities and energy densities; however, a higher charge voltage causes oxidative decomposition of the electrolytic solution at the positive electrode or degradation of the positive electrode.
In order to solve these problems, several attempts have been made including using a highly oxidation-resistant electrolytic solution or coating the surface of the positive electrode.
Japanese Patent Laid-open of PCT Application No. 2007-504628 uses an electrolytic solution containing fluoroethylene carbonate (FEC), which is obtained by replacing some of the hydrogen atoms of ethylene carbonate by fluorine. This document states that the use of FEC can form a thermally stable film at the negative electrode, thereby improving the thermal stability of the battery.
Japanese Patent Laid-open No. 2007-305546 discloses a technique to improve battery characteristics such as cycle characteristics by coating zirconia or other inorganic oxides on the surface of the positive electrode.
Japanese Patent Laid-open No. 2005-196992 proposes using a positive-electrode active material composed of a layered composite oxide containing Li, Ni, Mn, Co, and at least one rare earth element.
In spite of these attempts, however, the batteries decrease in remaining capacity and returned capacity when subjected to continuous charge at high voltages and high temperatures.